Telephone answering machines are well known. A typical consumer telephone answering machine is a stand-alone device coupled to a telephone line at a single location, and in parallel with one or more telephones coupled to the telephone line at the same or other locations. This type of answering machine typically includes circuitry for (1) detecting a ringing signal on the telephone line indicating the presence of an incoming telephone call, (2) answering the call by taking the line off-hook, (3) playing an outgoing or "greeting" message, (4) recording an incoming message, and (5) hanging up the line (going back on-hook) in order to respond to a subsequent telephone call. The greeting and incoming recorded messages typically are recorded in the analog domain using one or more conventional audio tapes. Answering machines of this type also are known in which either or both of the greeting and incoming messages are recorded instead in the digital domain using RAM or other storage device.
In addition to the foregoing, stand-alone answering machines commonly include circuitry for enabling remote-control operation of various ones of the machine's functions (e.g., playback of recorded messages, fast forwarding and rewinding of recorded messages, recording a new greeting message, and so forth). Such remote-control operation typically relies on the use of a conventional dual-tone multi-frequency (Touchtone.RTM., or "DTMF") telephone (or DTMF-tone generating device) remotely located off of the premises at which the answering machine is located. Typically, such remote-controlled answering machines include circuitry for detecting DTMF signals generated by or from the remote telephone, and circuitry for responsively controlling the answering machine's operations. To guard against unauthorized access to recorded messages, a remote user typically must first transmit to the answering machine a sequence of DTMF signals, indicative of a predetermined security code, before the machine will respond to remotely generated DTMF command signals. An example of such a stand-alone, remote-controlled answering machine is the model TAM-50, available from Sony Corporation. There are many other examples of similar answering machines on the market.
Other types of answering machines, called "voice mail" systems, are also well known. Voice mail systems serve a plurality of users each having his or her own individually controlled and accessible "voice mailbox." Each voice mailbox may be accessed from substantially any other remotely located telephone, either within or without the dwelling. Because of their ability to service multiple users, voice mail systems are found typically in offices and other commercial environments to provide a centralized telephone answering and messaging function. While once very expensive and thus found only in commercial settings, voice mail systems today may be found small office or even residential environments having an IBM or IBM-compatible personal computer ("PC"). PC-based voice-mail systems typically comprise a circuit card placed in an expansion slot of the computer that in turn is coupled to one or more telephone lines via "ports" on the card.
Even though stand-alone telephone answering machines exist that may be remotely-controlled, as described above, known machines suffer from a common problem. This problem is that remote-control operation may be achieved only by dialing into the machine from another telephone coupled to another telephone line within or without the dwelling in which the answering machine is located. That is, known stand-alone telephone answering machines may not be remotely controlled by another telephone coupled to the same telephone line within the dwelling. As a result, whenever it is desired to obtain messages from the answering machine (for instance, while at home), it is necessary to go to the answering machine to play back recorded messages or otherwise control its operation using the machine's control panel. At best, this is inconvenient.
One advantage that voice-mail telephone answering machines have over stand-alone telephone answering machines, when coupled to a PBX or to a Key Telephone System, is that voice-mail messages may be retrieved by remote control from substantially any telephone within or without the dwelling in which the voice-mail machine is located. This capability exists because the PBX or Key Telephone System is placed in effective series between the dwelling's telephone lines and the public telephone network, and controls (i.e., can prevent) access to the network (also called, herein, Central Office Lines). Thus, a PBX or Key Telephone System allows dialing from one telephone extension within a dwelling to another (such as to the voice-mail system to retrieve recorded messages) without interference from signals sourced from the telephone network (because the extensions in use for retrieving voice-mail messages are not coupled at that time to the telephone network).
While PBX or Key Telephone voice-mail systems work very well, a problem with such an approach to implementing a telephone answering function is that it is expensive. Moreover, because a PBX or Key Telephone System approach to implementing a telephone answering function requires placing circuitry in series between the public telephone network and the telephones (extensions) within the dwelling, such a system can be inconvenient or difficult to install (particularly in a house or apartment).